This invention relates to a measurement probe for solid state devices and, more particularly, to a measurement probe having a power detector for constant power microwave or millimeter wave device measurements. Microwave or millimeter wave integrated circuits require measurement probes that are designed specifically for microwave or millimeter wave purposes. A common measurement probe used for microwave or millimeter wave circuits comprises a dielectric substrate, a coplanar waveguide and a structure to which the substrate is attached. The coplanar waveguide typically includes a central coplanar waveguide signal line and two adjacent coplanar waveguide ground planes. Such measurement probes are well known in the art.
FIGS. 1 and 2 illustrate a typical prior art measurement probe. FIG. 1 illustrates a side view of measurement probe 8. The measurement probe has a microwave coaxial input connecter 10, a probe body 12, a probe substrate 14, a probe tip 16, a coaxial to coplanar waveguide transition connection 18, and a coplanar waveguide surface 20. FIG. 2 shows a bottom view of measurement probe 8 illustrating the detail of the coplanar waveguide surface 20. Coaxial to coplanar waveguide connection 18 connects to coplanar waveguide signal line 24. Coplanar waveguide signal line 24 extends from connection 18 to probe tip 16. Flanking coplanar waveguide signal line 24 are coplanar waveguide ground planes 26 and 28. Measurement probes such as shown in FIGS. 1 and 2 are commercially available from a variety of sources including, for example, Tektronix, Inc. and Cascade Microtech, Inc.
FIG. 3 shows a block diagram of a typical test configuration using a microwave measurement probe. A microwave or millimeter wave source 30 is used to generate a signal to be applied to a device, such as an integrated circuit 34. The wave is transmitted through coaxial cable 32 and measurement probe 8 to integrated circuit 34. One problem with such test configuration is that the power output of source 30 will often vary as the frequency of source 30 varies, as measured at the point where probe 8 contacts integrated circuit 34. In the past, this power variation problem has been addressed by connecting discrete power detecting circuits at the output of source 30 between source 30 and coaxial cable 32 and feeding back signals from the power detecting circuits to source 30. Then, the power emitted from source 30 may be changed in response to the detected signal so that a constant power is emitted at the source as the frequency changes. Such power detecting circuits often include a combination of coaxially connected directional couplers and coaxially connected Schottky detector diodes, or (at higher frequencies) a combination of rectangular-waveguide directional couplers and rectangular-waveguide-mounted Schottky detector diodes.
U.S. Pat. No. 5,003,253 to Majidi-Ahy et al. is an example of measurement probes that contemplate both diodes and directional couplers. However, in Majidi-Ahy et al. the diodes are used as nonlinear elements for harmonic generation to optimize harmonic signal generation. Thus, circuitry for power level detection and control is not proposed.
It is desirable to more accurately maintain a constant power output to an integrated circuit device than is obtained with the probe systems described above. Furthermore, it is desirable to maintain such constant power output with a simplified approach that does not require additional connections of coaxially coupled or rectangular-waveguide-coupled power detector systems.